The present invention relates to a method of determining electrical currents to be used in active shim coils in magnetic resonance magnets to maximize the magnetic field homogeneity in the bore of the magnet.
In magnetic resonance (MR) magnets a uniform magnetic field is used to polarize the hydrogen nuclei located in the subject being scanned. Magnetic field inhomogeneities will distort the position information in the scan volume and degrade the image quality. In chemical shift spectroscopy the chemically shifted frequency peaks are often separated by a fraction of one part per million, requiring high field homogeneity. To create a highly uniform magnetic field with an electromagnet, it is necessary to build the magnet to a carefully specified shape, and to strive to minimize the deviations from the specified shape due to manufacturing variations. The resulting magnet, however, typically requires field correction to achieve the desired level of homogeneity, due to deviations of the magnet from the design or due to the presence of ferromagnetic material in the vicinity of the magnet.
To improve field uniformity, correction coils are typically used. These coils are capable of creating different field shapes which can be superimposed on an inhomogeneous main magnetic field to perturb the main magnetic field in a manner which increases the overall field uniformity. Many sets of such coils are typically required. The state of the art magnetic resonance imaging magnet has between 10 and 20 independent sets of correction coils, each with its own power supply to provide the correct current flow. The correction coils may be resistive, superconducting or a combination of both. Correction coils are generally designed to produce certain terms of a spherical harmonic field expansion. By energizing the coils at a proper combination of currents, a homogeneous field can be achieved within the imaging volume.
Presently, the currents in coils positioned in the bore of the magnet and placed to eliminate selected harmonics are determined by decomposing the magnetic field produced by the magnet into several terms of a spherical harmonic expansion and predicting coil currents which eliminate these harmonics thereby increasing the homogeneity of the field.
There are several problems in using selective harmonic elimination. One of the problems occurs when the magnetic field is corrected for the wrong harmonics. This can occur because of an undersampling of the magnetic field and/or low order fitting. Undersampling does not allow proper determination of the harmonic components that make up the field, particularly the high order harmonics. Low order fitting may not eliminate the high order harmonics which can significantly contribute to the inhomogeneity. The low order correction coils while generating significant amounts of the harmonic they were designed to produce, also generate other higher order harmonics. The determination of the currents of the correction coils when using harmonic elimination requires many iterations resulting in long computation times, sometimes generating solutions which cannot be used because of the current limitations of the correction coil power supplies.
A linear least squares approach described and claimed in U.S. Pat. No. 4,771,244, minimizes the average field tolerances. Two disadvantages with this method are that it is not a peak to peak minimization and therefore only indirectly reduces the peak to peak field inhomogeneity. In MR magnets inhomogeneity is specified in parts per million peak to peak. Secondly, there are no constraints in the linear least squares method on the predicted shim currents which from time to time result in predicted currents which surpass the limits of the shim coil power supplies. When the predicted shim currents exceed the power supply capability, iron shims need to be used in combination with the correction coils.
It is an object with the present invention to provide a method of shimming magnets using correction coils with correction coil currents that do not exceed any of the correction coil power supplies capabilities and which minimize global peak to peak magnetic field inhomogeneity.